Electrodeposition coating processes have been a widely used for applying anti-corrosive primer coatings to various metallic substrates such as automobile bodies because they are applicable to a variety of substrates regardless of shape or configuration and also hazardless to attendant workers and to the environment. Coating compositions used in the electrodeposition processes are in the form of an aqueous solution or dispersion of electrically charged resin particles. Accordingly, their stability and throwing power are important for the consistent operation of an electrodeposition coating line. With regard to finished coating films, it is desirable to have a large film thickness and high performance properties in such a field as automobile painting where a high corrosion and chipping resistance is essential. Furthermore, there is ever growing needs among users for coated articles having a highly aethetic appearance in terms of sharpness and gloss.
It is known to incorporate internally crosslinked polymer microparticles into electrodeposition coating compositions. Japanese Laid Open Patent Application 8-93762 discloses an electrodeposition coating composition containing an emulsion of internally crosslinked copolymers of a polyfunctional monomer and a monofunctional monomer dispersed in a solution of a water-soluble thermosetting resin. This composition gives a coating film having a irregular surface due to the presence of polymer microparticles embedded therein so that irregular reflection takes place on the surface to give a mat finish. Obviously, it is not intended that the resulting film to be provided thereon with further coatings such as middle and/or top coatings. A similar coating composition based on the same principle for decorating aluminum or aluminum alloy substrates with a mat finish is also disclosed in Japanese Laid Open Patent Application 56-49766.
These known compositions have certain disadvantages in that they have a limited use only for imparting a mat finish on a glaring surface and are not adapted for universal uses. The polymer microparticles in the liquid composition tend to settle upon storage and have an adverse effect on the electrodeposition operation when added in amounts sufficient to give a satisfactory mat effect.